Watercraft are often powered by an inboard or outboard motor. The motor includes a water propulsion device, such as a propeller, which is powered by an internal combustion engine. The engine has an output shaft which drives the water propulsion device.
In the "planing-type" watercraft, the watercraft moves from a position low in the water at a low speed to a position high in the water at a higher speed. When the watercraft is accelerated from the low speed position to the planing position, a large load is placed upon the engine. After the watercraft has moved to a planing condition, less of the watercraft contacts the water, reducing associated drag. This arrangement is illustrated in FIG. 6.
An ignition arrangement associated with an engine powering such a watercraft is illustrated in FIG. 5(a). An output signal is received from a mechanism which provides a signal dependent upon the speed of the engine. Generally, the output signal is generated by a pulser coil associated with the rotating crankshaft of the engine. In this arrangement, time Tn is the time between successive output signals and represents the time it takes for the crankshaft to complete one revolution.
The ignition elements associated with the engine are fired at a firing time obtained from a map based on engine speed. At least one ignition element is fired at a time corresponding to crank angle .beta.. Since the crank angle is not known exactly at all times, the position .beta. is estimated by determining the time t it takes for the crankshaft to rotate to position .beta. at the speed of the engine. The ignition control calculates time t based on time Tn, and then outputs the appropriate firing signal.
In the situation where the engine speed is relatively steady, as in FIG. 5(a), this arrangement is fairly suitable. Where the engine speed is rapidly changing, such as in the case where the engine speed is accelerating to move a watercraft from a low speed to a planing position, this method of calculating the firing timing is detrimental. In particular, when the engine speed increases, the time Tn during each successive interval (i.e. Tn, Tn+1, Tn+2) becomes successively shorter. Therefore, if the firing timing for the next interval is calculated based on the time from the previous interval, the ignition firing timing is much too late as compared to the desired firing timing. Conversely, if the engine speed reduces quickly, the firing timing interval is based on a time which is too short compared to the next interval, and the firing timing is advanced in relation to the desired or optimum firing timing.
During the transition of the watercraft to a planing condition, the firing timing delay results in a loss in power, slowing the speed of watercraft planing. Conversely, when the watercraft moves from a planed to a low speed condition and the load decreases, the firing timing advance results in an unnecessarily high reduction in engine speed.
As one method to overcome the above-stated problem, the ignition timing may be calculated by estimating the rate of change in engine speed from two successive crankshaft rotations. This arrangement, however, slows the responsiveness of the ignition control. In addition, where the engine speed is rapidly fluctuating, the estimate of the future engine condition may vary substantially from the actual engine condition at the time the firing signal is actually output.
An engine control is desired which overcomes the above-stated problems and which provides for smooth transition of the watercraft to and from a planed condition is desired.